Tungsten wire reinforced silicon nitride articles and method for making the same

ABSTRACT

A reinforced ceramic article suitable for use in a gas turbine engine environment comprises a matrix material of Si3N4 with reinforcing filaments of tungsten embedded therein. The article has improved impact strength and thermal shock resistance. A method for making the article is also disclosed.

Brennan et al.

United States Patent 1191 1 Oct. 21, 1975 TUNGSTEN WIRE REINFORCEDSILICON NITRIDE ARTICLES AND METHOD FOR MAKING THE SAME [75] Inventors:John J. Brennan, Portland; Richard C. Novak, Glastonbury; Michael A.

De Crescente, Wethersfield, all of Conn.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn. I

[22] Filed: Sept. 4, 1973 [21] Appl. No.: 394,433

[56] References Cited UNITED STATES PATENTS 3,427,185 2/1969 Cheatham etal 117/128 3,766,000 10/1973 Gibson et al 161/170 FOREIGN PATENTS ORAPPLICATIONS 1,646,666 1/1972 Germany 264/56 2,010,322 2/1970 France264/56 41-10337 6/1966 Japan 264/60 954,285 2/1964 United Kingdom...264/60 331,047 6/1970 U.S.S.R 264/60 Primary ExaminerGeorge F. LesmesAssistant Examiner-Stanley S. Silverman Attorney, Agent, or Firm-Jenn D.Del Ponti [5 7 ABSTRACT A reinforced ceramic article suitable for use ina gas turbine engine environment comprises a matrix material of Si ,Nwith reinforcing filaments of tungsten embedded therein. The article hasimproved impact strength and thermal shock resistance. A method formaking the article is also disclosed.

6 Claims, 1 Drawing Figure US. Patent Oct. 21, 1975 3,914,500

for'use' in a'gas turbineen'gine. 1

Another object dfjthepresent invention -is a 'fiber- 'IUNGS'IIEN..-WIR-EREINFORCED SILICON NITRIDE ARTICLESAND METHOD FOR MAKING 1 v TI-IE SAMEBACKGROUND OF THE INVENTION Q 1. Field of the Invention This inventionrelates xto fiber-reinforced ceramic matrix composites. v

2. Description of the Prior Art Ceramic materials, by virtue of theirhigh melting points and oxidation resistance", can offer large gains ingas turbine performance, provided some serious limitations can'beovercome. Theseliniitations'are poor thermal-I shock' resistance and,more critically, low impact strength. It is recognized in the prior artthat ceramic matrices reinforced with filaments of various materials mayhave good impact strength. For example, US}. Pat.

No. 3,386,840 to Gruber suggests that fibers ofsiliconcarbide'va'ryingin length from about 250 Angstrorns to 1 inch maximum, may be usedto'improve the strength of refractory metals such as nitrides. In column8 example -7, Gruberindicatessthat sillimanite (A1,0,.. SiO,) containing10 percent of these relatively short fibers of silicon-carbide .wasfound toizhave-a greatenimpact strength than the ordinary-sillimanitewithout this additive. Cheatham et al; U .SaPat. No.3,427,185,.suggestsforming a composite structural material by plasma arcsprayingror vapor depositing the matrixmaterial over a mat offilamentary material. In column.6' he lists a largenumber of filamentarymaterials, suchas 'tungsten,: and matrixa materials, including som'eccramics, which :he suggests may be combined'in the manner of hisinvention; howe-ver, his-only example is thatof using high carbon steelwires in a matrix of aluminum. It cannot be determined from the-patentwhether'anyother combinations'were tried and'jwhat results wereobtained. v Z w :Despite the various combinations of matrices, includingceramic matrices and fiber reinforcements discussed in the prior artnone-to date have-proven entirely satisfactory for use' in "gasturbineengines, and more parti'cularly for use as bl-ades'or vanes in-gas turbine eng-ines.- Despite the claims foflhigher impact strength,the impact strengthof priorart ceramic ma- SUMMARY oFTi EiNvENTioNAnobject "of the .prescnt invention is 'a fiberreinforced ceramic matrixcomposite material suitable reinforced ceramic matrix composite having arelativeiy high"i""pac t strength. and thermal shock resistance. w

Accordingly," the" present ,inyention is f a fiber- -reinforced ceramicarticle comprisingldcnsified hotpressed silicon nitride (Si N reinforcedwithcontinuwas filaments l 'oft'ungsteniw).

t has been found by C-harpy impact'teistirig that'this .rnate'rial,' athigh temperaturessuch asthosefencount'ered' in a'gas" turbine engine, isable to absorb many times theenergy of 'urire'inforced hot-presseds'ili'con nitride. Ability to withstand thermal shock is alsosignificantly improved. I Y

iii-brie embodiment si,N.;+ 5% MgO with tungsten filaments'wasjhot-pressed' at" l650C', 4000 psi for l h'ouflTe'st s ecimens weremade in this manner using shock resistance, creep strength, and impactresistance.

. Man y ceramic materials meet the criteria of strength and oxidatiorilresistance"at the temperature of interest,

which in our caseisabout 2400?, butdo not have good thermalshock;lresistance. The refractory oxides fall in this category. Forexarnple, zirconium diboride and silicon carbide are strong andoxidation resistant materials. Zirconium diboride, however, does notpossess very good thermal shoclg resistance and also re- 3 quirestemperatures on the order of 21 00C, for fabrication, which for anumb erof reasons is much too,high

for successful incorporation, ofreinforcingfibers, as

will-be further discussed..Silicon carbide must also be hot vpressedabove 2000C for complete densification ,wires.

, Many reinforcingfibers were considered for use with the.siliconnitride matrix, however, of those initially considered, none werefound satisfactory except tungstem For example, it isdesirable that .thecoefficientof thermal expansion of a filament. used in reinforcing aceramic matrix be greater than that of the matrix so thaton cooling fromthe :fabrication temperature the vceramic matrix isput into a'compressiveprestress.

Thus, when Al O fibers are used in a silicon nitride "matrix, their veryhigh coefficient of thermal expansion results in extremelylarge$prestresses in the fibers such that tensile failures occur oncooling-from the fabrication'temperatu're: For-this reason A1 0 fibersare not acceptable. In, this regard, silicon carbide fibers (SiC)weresuccessful inputting a compressive prestress in the ceramic matrix;however, during the hot-pressing operation at 1650C a reaction tookplace within the fiber; between its tungsten core and the siliconcarbide,

' befound.

forming tungsten silicide which reduced the ultimate tensile strength ofthe're'inforcing silicon carbide fiber 'plications under consideration;Attemptswere madeto hot-press the "to'below acceptable levels making ituseless for the apsilicon nitride at lower temperatures to'avoid theundesirable reactions which take place with'varioustyp'es of reinforcingfilaments. However, with MgO as the densification aid, satisfactorydensification does not occur below 1600C. Other satisfactorydensification aids could not Attempts were made 'to find coatings forthe filaments which would prevent or reduce unfavorable reactions at'the high hot-pressing temperatures, but'all tagesof thepresentinvention will become moreappar ent in, the -light of thefollowingdetailed description of preferred embodiment thereof and asillustrated in the accompanying drawing.

BRIEF "DESCRIPTION OF' THE DRAWING Q FIG; 1 is graph wherein the thermalshock resistrance of tungsten reinforcedSi N,'rnay be compared to thethermal shock resistance of unreinforced Si N,.

' DESCRIPTIONOF'THE PREFERRED EMBODIMENTS In one embodiment "of thepresent invention, the composite structureis produced from a layup ofseveral plies sheets. Each sheet comprises 25 volume percent tungstenwires slurry coated with a mixture of Si N plus 5%byweight M gO powderfor densification and including a binder, which inthis preferredembodinient'is polystyrene dissolved in toluene, which cures at ro'omtem'perature. These plies or sheets are stacked to a desired thicknessand then hot-pressed -at l650C, 4000 psi f or 1 hour or at least untilfull densification of the matrix isap proached. Th eresulting structureof this preferred embodiment is over 99% dense. During the hot-pressingoperation a layer of tungsten silicide forms around each wire whichbonds the wires to the matrix material. As a result of this'bond thewires are able to induce a compressive'stress in the ceramic," which isrequired'for engine applications. I

C'are'istaken t'o assure that the finished article does not have 'anyexposed tungsten wi'res. This is desirable in the corrosive environmentof a gas turbine engine but not necessarily-mandatory sinceat hightemperatures a protective-coating forms on the surfaceof exposedtungsten wires. Satisfactory test specimens were made using tungstenwires having diameters ranging from 5 to 10 mils. Satisfactory testspecimens were also fabricated with a volume percent of tungsten wireswithin the range :of from l5 to'45%.lt is by no means intended tolimitthe present'invention to within these ranges.

-ln .this preferred embodiment the tungsten wires in each sheetarexcontinuous-and parallel and the wires within adjacent sheetsareparallel to each other; however, very satisfactory results may beobtained by having the wires of adjacent. sheetsat an angle with respectto each other to form a diamond shaped pattern.

-Test-';samples were made up-according to the above TABLE! v o: Si N anda SEN W Composite Properties 01 SEN. a Sig 5% MgO Property 5% MgO 25Vol% W Modulus of Rupture Room TempllO-Ksi 75 Ksi TABLE I-Continued' aSi N and a Si N 4 W Composite Properties I a Si N a Si,N 5% MgO Property5% MgO 25'Vol% W 1400C 20 Ksi No data Charpy Impact Room Temp .45 ft-lb.50 ft-lbs. l300C .40 ft-lb l.20 ft-lbs.

TABLE II B SEN, and B Si N W Composite Properties B Si3N B Si,N, 5% MgOProperty 5% MgO 25 Vol% W Modulus of Rupture I Room Temp 72 Ksi 95 Ksil400C 23 Ksi 40 Ksi Charpy Impact I Room Temp .49 ft-lbs. .5l ft-lbs.

l300C .25 ft-lbs. 2.25 ft-lbs.

From Table I it can be seen that a Si N tungsten reinforced compositehas three times the impact strength of its unreinforced counterpart atl300C. From Table II it can be seen'that B Si N tungsten reinforcedcomposite structurehad nine times the impact strength of itsunreinforced counterpart.

- :From the foregoing it can be seen that both a and B hot-pressedsilicon nitride reinforced with tungsten wires have greatly improvedimpact strength at high temperatures, the B silicon nitride matrix beingpreferred. In the claims theuse of the term Si N without i an a or Bprefix is intended to include either a Si N or B Si;,N,.

The graph and data displayed in FIG. 1 show that tungsten reinforced SiN offers significantly better resistance to thermal shock than doesunreinforced bulk Si N,. The solid line represents the calculatedthermal stress, as .a function of quench temperature, which would resultfrom. quenching small specimens of unreinforcedSi N in water.-.Thebroken line represents the calculated thermal stress for tungstenreinforced Si N The stress in the tungsten reinforced 'Si N is smaller,I primarily as a result of its higher thermal conductivity. Alsodisplayed on the graph of FIG. 1 are data points of room. temperaturemodulus of rupture values of specimens quenched from the indicatedtemperatures. The xs are data points for tungsten reinforced Si N 0 andthe'Os are data points for the unreinforced ,Si -,N

It is apparent that above quench temperatures of I200F the tungstenreinforced Si N is much stronger than the unreinforced Si -,N

Although the invention has been shown and described with respecttopreferred embodiments thereof, it should be understood by those skilledin the art that various changes and omissions in the form and detailthereof may bemade therein without departing from the spirit and thescope of the invention.

Having thus described typical embodiments of our invention, that whichwe claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An impact resistant fiber-reinforced composite article suitable foruse ina gas turbine engine environment comprising a plurality ofcontinuous tungsten wires preferentially oriented in a hot-pressed Si3N4matrix, which approaches full densification, each of said wires having alayer of tungsten silicide thereon, said tungsten silicide layer beingchemically bonded to said wires and said matrix and said matrix being incompression as a result of said bond.

5. The fiber-reinforced composite article according to claim 4 whereinsaid matrix material includes MgO as a densification agent.

6. The fiber-reinforcedcomposite article according to claim.4 whereinsaid Si N is [3 Si,N

l i l

1. AN IMPACT RESISTANT FIBER-REINFORCED COMPOSITE ARTICLE SUITABLE FORUSE IN A GAS TURBINE ENGINE ENVIRONMENT COMPRISING A PLURALITY OFCONTINUOUS TUNGSTEN WIRES PREFERENTIALLY ORIENTED IN A HOT-PRESSED SI3N4MATRIX, WHICH APPROACHES FULL DENSIFICATION, EACH OF SAID WIRES HAVING ALAYER OF TUNGSTEN SILICIDE THEREON, SAID TUNGSTEN SILICIDE LAYER BEINGCHEMICALLY 00BONDED TO SAID WIRES AND SAID MATRIX AND SAID MATRIX BEINGIN COMPRESSION AS A RESULT OF SAID BOND.
 2. The fiber-reinforcedcomposite article according to claim 1 wherein said matrix includes MgOas a densification agent.
 3. The fiber-reinforced composite articleaccording to claim 1 wherein said Si3N4 is Beta - Si3N4.
 4. An impactresistant fiber-reinforced composite article suitable for use in a gasturbine engine environment comprising a plurality of plies bondedtogether, each ply including a plurality of continuous tungsten wirespreferentially oriented in a hot-pressed Si3N4 matrix, which approachesfull densification, each of said wires having a layer of tungstensilicide thereon, said tungsten silicide layer being chemically bondedto said wires and said matrix and said matrix being in compression as aresult of said bond.
 5. The fiber-reinforced compositE article accordingto claim 4 wherein said matrix material includes MgO as a densificationagent.
 6. The fiber-reinforced composite article according to claim 4wherein said Si3N4 is Beta - Si3N4.